Aramid/epoxy composites sandwich structures for low-observable radomes

Low-observable radomes are usually made of E-glass/epoxy composite due to its low dielectric constant which is necessary not to interfere electromagnetic (EM) wave transmission characteristics. Since aramid fibers have lower dielectric constant and higher strength than those of E-glass fiber, aramid fiber radome structures may have better the EM transmission and mechanical characteristics than those of E-glass/epoxy radomes. In this work, the low-observable radome was constructed with a sandwich construction composed of aramid/epoxy composites faces, foam core and Frequency Selective Surface (FSS) which had the abilities of transmitting EM waves selectively in the X-band range. The EM wave transmission characteristics of the low-observable radome were simulated by a 3-dimensional electromagnetic analysis software and also measured by the free space measurement method with respect to the pattern size of FSS and foam cores. The mechanical properties of the low-observable radome made of aramid/epoxy composite were measured by the 3-point bending test and compared to those of the conventional low-observable radome made of E-glass/epoxy composite.     

Composite sandwich constructions for absorbing the electromagnetic waves

RAS (radar absorbing structures) is a key component for weapon systems such as aircrafts, warships, and missiles to achieve both the stealth performance by absorbing EM (Electromagnetic) waves incident on and load bearing capability. In this work, the RAS was fabricated as sandwich constructions composed of nanocomposite, carbon fabric/epoxy composite, and PVC foam. The nanocomposite composed of E-glass fabric, epoxy resin, and CNT (carbon nanotube) was adhesively bonded to the outside of the sandwich construction in order to absorb EM waves. The carbon fabric/epoxy composite had the dual roles as the reflection layer of incident EM waves and load bearing face material of sandwich constructions. Using the fabricated sandwich constructions, the EM absorbing characteristics were measured by the free space measurement system and the bonding characteristics between nanocomposites and carbon fabric/epoxy composites also were investigated.     

Development of rigid bio-based polyurethane foam reinforced with nanoclay

Integration of organic nanoclay into bio-based polyurethane (PU) foam is a promising alternative to enhance the foam’s properties via green technology. In this paper, modified diaminopropane montmorillonite (DAP-MMT) nanoclay was introduced into palm oil-based PU foam at different weight loadings, namely, 0, 2, 4, 6, 8, and 10 wt.%, in order to investigate the effects on the mechanical and thermal properties of the foam. Several tests and characterizations were carried out to study the surface morphology, density, compressive strength and thermal stability of the foam. It was found that foam exhibited an exfoliated or intercalated microstructure based on the DAP-MMT contents. The X-ray diffraction analysis showed that below 4 wt.%, the foams displayed exfoliated structures while beyond the value, the foams exhibited the intercalated morphologies. Closed cells with different cell sizes were observed when the DAP-MMT contents were varied. Meanwhile, thermal stability and compressive strength of foams increased with increasing DAP-MMT contents up to 4 wt.%, as shown by thermogravimetry analysis and compression test, respectively.     

Buckling,collapse and failure analysis of FRP sandwich panels

Rectangular orthotropic fiber-reinforced plastic (FRP) sandwich panels were tested for buckling in uni-axial compression. The panels, with 0.32 cm (0.125 in.) face sheets and a 1.27 cm (0.5 in.) core of either balsa or linear poly(vinyl chloride) (PVC) foam, were tested in two sizes: 154×77 cm² (72×36 in.²) and 102×77 cm² (48×36 in.²). The sandwich panels were fabricated using the vacuum-assisted resin transfer molding process. The two short edges of the sandwich panels were clamped, while the two long edges were simply supported for testing. The clamped panel ends were potted into a steel frame. The experimental elastic buckling loads were then measured using strain gauges fixed to both sides of the panels. A total of 12 panels were tested under uni-axial compression. Bifurcation in the load versus engineering strain curve was noted in all cases. For all six sandwich panels tested using balsa core, the type of failure was easily identified as face sheet delamination followed by core shear failure. For all six PVC foam core sandwich panels tested, the type of failure consisted of core shear failure with little or no face sheet delamination. In the failed balsa core panels there was little or no evidence of balsa remaining on the FRP face sheet, however, in the PVC foam core panels there were ample amounts of foam left on the FRP face sheet. It was concluded that although the buckling loads for the foam core panels were not as high as those for the balsa core panels, PVC foam core bonding to the FRP face sheets was superior to balsa core bonding.     

Broadbanding of A-sandwich Radome Using Jerusalem Cross Frequency Selective Surface

Enhancement of electromagnetic performance of A-sandwich radome using aperture-type Jerusalem cross frequency selective surface (FSS) is presented. The Jerusalem cross FSS array is embedded in the mid-plane of the core of Asandwich radome to enhance the EM performance parameters over the entire Xband. For modeling the Jerusalem cross FSS embedded radome panel and evaluation of its EM performance parameters, equivalent transmission line method in conjunction with equivalent circuit model is used. A comparative study of Jerusalem cross FSS embedded A-sandwich radome and A-sandwich radome of identical material and thickness (core and skin layers) indicate that the new wall configuration has superior EM performance as compared to the A-sandwich wall alone configuration. The excellent EM performance of Jerusalem cross FSS embedded A-sandwich radome makes it a desirable choice for the design of normal incidence radomes (hemispherical/ cylindrical), near-normal incidence radomes (paraboloidal) and highly streamlined airborne nosecone radomes.     

Graded Dielectric Inhomogeneous Planar Layer Radome for Aerospace Applications

Controllable artificial dielectrics are used in the design of radomes to enhance their electromagnetic (EM) performance. The fabrication of such radome wall structures with controllable dielectric parameters seems to be an arduous task. Further even minor fluctuations of dielectric properties of radome wall due to fabrication uncertainties tend to result in drastic degradation of radome performance parameters. In the present work, a novel inhomogeneous radome with graded variation of dielectric parameters is proposed which limits the constraints on fabrication and facilitates excellent EM performance characteristics. This radome wall consists of five dielectric layers cascaded such that the middle layer has maximum dielectric constant and electric loss tangent. The dielectric parameters of the layers on both sides of the middle layer decrease in a graded (or step-wise) manner. The EM performance characteristics of the IPL radome with graded dielectric parameters are superior to that of conventional monolithic half-wave radome.     

A Novel Metamaterial FSS-based Structure for Wideband Radome Applications

A novel metamaterial based FSS (frequency selective surfaces) structure is presented in this paper for wideband airborne radome applications. The proposed metamaterial-FSS structure consists of three layers, where a DPS (double positive sign) layer is sandwiched between a MNG (μ-negative) and ENG (ε- negative) layer, exhibits very good bandpass characteristics inside the operational band along with excellent roll-off characteristics outside the band. The EM performance analysis of the proposed structure has been carried out using transmission line transfer matrix (TLTM) method, which shows excellent bandpass characteristics over a wide frequency range. The transmission efficiency is over 95% both at normal incidence and at high incidence angles of 30°, and 60°. The frequency range extends from S- to X-band (2.5-9.9 GHz). In view of streamlined airborne radome applications, the reflection properties and insertion phase delay (IPD) are also determined at high incident angles.     

Characterization of fracture toughness (<Emphasis Type="Italic">G</Emphasis><Subscript>c</Subscript>) of PVC and PES foams

The fracture behavior of polyvinyl chloride (PVC) and polyethersulfone (PES) foams has been examined using the single-edge notch bend and the double cantilever beam (DCB) tests. PVC foam densities ranging from 45 to 100 kg/m³ and PES foam densities ranging from 60 to 130 kg/m³ were examined. The PVC foams failed in a linear elastic brittle manner, whereas the PES foams displayed much more ductility and substantially larger toughness at a comparable foam density. The cell wall thickness of the PES foams was almost twice the thickness of the PVC foams which may have contributed to the high fracture toughness here defined as critical energy release rate (G _c). The PES foam, further displayed low initiation toughness, due to the sharp artificial crack tip and large toughness corresponding to propagation from a natural crack. The results show that the ductile PES foams have toughness close to its solid counterpart whereas the toughness of the PVC foams falls substantially below its solid counterpart.     

Damping characteristics of nanoclay filled hybrid laminates during medium velocity impact

The objective of this paper is to study the vibrational damping characteristics during medium velocity impact of nanoclay filled glass fiber reinforced epoxy hybrid laminates. A series of laminates with varying degree of nanoclay concentration (0–5 wt.%) and fiber weight fraction (25–75 wt.%) were prepared by vacuum assisted resin infusion molding (VARIM) method. The laminates were subjected to medium velocity projectile impact using in-house built gas gun set-up and the ballistic limit of laminates series was determined. The result indicated that during impact, the laminate undergoes vibrational damping. This damping property is a function of fiber weight fraction and orientation, nanoclay concentration and nanocomposite structure. A 42% increase of ballistic limit was observed for 5 wt.% nanoclay filled hybrid (50 wt.% fiber) when compared with unfilled composite. Structural and modal analysis of hybrids showed that the increased ballistic limit of nanoclay filled hybrids is due to the nanocomposite structure and improved damping and fracture properties.     

Long term water uptake of a low density polyvinyl chloride foam and its effect on the foam microstructure and mechanical properties

The water uptake, evolution of the cell morphology and basic mechanical properties of a 48 kg/m³ commercial polyvinyl chloride (PVC) foam immersed in distilled water and seawater for up to 12 months is investigated. The samples of PVC foam immersed in distilled water showed a faster water absorption rate and water uptake than the samples immersed in seawater. For both conditions, the tensile and compressive properties of the foam evidence a plasticization effect with a small reduction in the elastic modulus (∼10%) and an increase in the ultimate tensile strain (∼19%) for 12 months of immersion. The detailed micrographic analysis conducted provides conspicuous evidence that for both conditions the cells at the surface of the foam are severely damaged after a few days of immersion, but such cell damage is superficial and does not cause severe irreversible damage to the internal cellular microstructure of the foam.     

Effect of coupling agent and nanoclay on properties of HDPE,LDPE, PP,PVC blend and Phargamites karka nanocomposite

High density polyethylene (HDPE), low density polyethylene (LDPE), polypropylene (PP) and poly(vinyl chloride) (PVC) were solution blended by using a mixture of xylene and tetrahydrofuran as solvent and polyethylene-co-glycidyl methacrylate (PE-co-GMA) as compatibilizer. The minimum ratio of solvents to obtain a homogenous solution was optimised. Wood polymer composites (WPC) were prepared by using solution blended polymer, wood flour and nanoclay. X-ray diffraction studies of WPC treated with 1 and 3 phr nanoclay showed higher exfoliation compared to WPC treated with 5 phr nanoclay. TEM study also supported the above findings. FTIR studies indicated an interaction between wood, PE-co-GMA and clay. SEM study indicated an increase in miscibility among polymers due to addition of PE-co-GMA as compatibilizer. Thermal stability improved on addition of clay to the WPC. WPC treated with 3 phr clay showed highest mechanical properties. Hardness and water absorption were improved significantly with the addition of nanoclay to wood/polymer composite.     

Water immersion effect on swelling and compression properties of Eco-Core,PVC foam and balsa wood

Syntactic foam, balsa wood and PVC foams are commonly used as core materials in sandwich structures for weight critical applications such as aircraft and ship structures. Water absorption is highly undesirable in these applications. The present study evaluates the effect of water immersion on three types of core materials, namely, Eco-Core, balsa wood and PVC foam. Eco-Core is a new fire resistant core material under development that utilizes about 83% by weight of fly ash. Designers of naval ships and aircraft commonly specify balsa wood and PVC foam as core materials for sandwich structures. These three core materials were subjected to water immersion to determine the relative resistance to property change. Both tap water as well as seawater was used. Core samples were studied for dimensional change, weight gain and compression properties after water immersion and the results were compared with the test results of dry core samples. Time periods included in the study ranged from 4 h to 500 days. The results showed that Eco-Core is as good as PVC foam in resisting swelling, water absorption and changes in compression properties due to water immersion. Where as balsa wood showed a significant swelling, water absorption and deterioration of compression properties.     

无反射层泡沫夹层结构设计及吸波性能研究EI北大核心CSCD

针对无反射层的电磁隐身需求,本工作对透波层/吸波泡沫/透波层的夹层结构的吸波性能进行仿真计算,据此制备不同电磁参数的吸波泡沫,对其进行电磁特性表征,并研究吸波泡沫夹层结构的雷达散射截面(RCS)性能。结果表明:在吸波泡沫介电常数为2.3~2.7,介电损耗为0.24~0.26时,无反射层的夹层结构在宽频范围内具有最优的吸波性能。加入炭黑吸收剂泡沫的介电常数和介电损耗具有明显的变化规律,吸波PMI泡沫的电磁特性与仿真计算最优吸波泡沫较接近。炭黑质量分数为8%时吸波PMI泡沫夹层结构在2~18 GHz频率范围内具有最优的隐身性能,与仿真计算结果相对应,其通过低频透波、高频吸波实现电磁波隐身。     

Radome health management based on synthesized impact detection,laser ultrasonic spectral imaging,and wavelet-transformed ultrasonic propagation imaging methods

A radome must not only withstand various forces during operation, but also provide a window for electromagnetic signals. A radome is generally a composite sandwich structure. Much of the damage to radomes is barely visible to the naked eye on the outer surface, but is severe internally. In this study, a radome health management strategy consisting of in-flight damage event detection and ground damage evaluation processes is proposed. A radome health management system, composed of an on-board subsystem and a ground subsystem, was developed to realize the strategy. An in-flight event detection system was developed based on acoustic emission (AE) technology. A built-in amplifier-integrated PZT sensor was used, and the minimum impact energy that the on-board subsystem can detect was determined. The AE sensor was then switched to an ultrasonic receiver. A scanning laser ultrasonic technology was combined with the ultrasonic receiver to develop a ground nondestructive evaluation subsystem. For in situ damage visualization, laser ultrasonic frequency tomography and wavelet-transformed ultrasonic propagation imaging algorithms were developed in this study. To demonstrate the robustness of the ground subsystem, a damage was generated by 5.42 J impact in a glass/epoxy radome with honeycomb core, and the impact image of 25 mm in diameter invisible outside could be visualized with the combination of ultrasonic spectral imaging (USI) and wavelet-transformed ultrasonic propagation imaging (WUPI), which made the propagation of only the damage-related ultrasonic modes visible.     

Resin infusion analysis of nanoclay filled glass fiber laminates

This paper focuses on the resin flow characteristics of nanoclay filled glass fiber laminates processed by Vacuum Assisted Resin Infusion Molding (VARIM). Laminates with varying quantities of nanoclays (0–5 wt.%) were prepared and the effect of these nanoclays on the epoxy resin flow characteristics was studied. It was found that the flow rate of resin continuously decreased as nanoclay content continuously increased. The reduction in the flow rate was attributed to the rate of change of curing and the subsequent change in viscosity of the nanoclay filled resin. Analysis of infusion process by Darcy’s law show that the permeability of the fiber decreased in the nanoclay filled resin system. Nanoclay filled laminates show improved static and dynamic mechanical properties than that of unfilled resin composites.     

Radome health management based on synthesized impact detection,laser ultrasonic spectral imaging,and wavelet-transformed ultrasonic propagation imaging methods

A radome must not only withstand various forces during operation, but also provide a window for electromagnetic signals. A radome is generally a composite sandwich structure. Much of the damage to radomes is barely visible to the naked eye on the outer surface, but is severe internally. In this study, a radome health management strategy consisting of in-flight damage event detection and ground damage evaluation processes is proposed. A radome health management system, composed of an on-board subsystem and a ground subsystem, was developed to realize the strategy. An in-flight event detection system was developed based on acoustic emission (AE) technology. A built-in amplifier-integrated PZT sensor was used, and the minimum impact energy that the on-board subsystem can detect was determined. The AE sensor was then switched to an ultrasonic receiver. A scanning laser ultrasonic technology was combined with the ultrasonic receiver to develop a ground nondestructive evaluation subsystem. For in situ damage visualization, laser ultrasonic frequency tomography and wavelet-transformed ultrasonic propagation imaging algorithms were developed in this study. To demonstrate the robustness of the ground subsystem, a damage was generated by 5.42 J impact in a glass/epoxy radome with honeycomb core, and the impact image of 25 mm in diameter invisible outside could be visualized with the combination of ultrasonic spectral imaging (USI) and wavelet-transformed ultrasonic propagation imaging (WUPI), which made the propagation of only the damage-related ultrasonic modes visible.     

Development of rubberized syntactic foam

This study explored a novel hybrid syntactic foam for composite sandwich structures. A unique microstructure was designed and realized. The hybrid foam was fabricated by dispersing styrene–butadiene rubber latex coated glass microballoons into a nanoclay and milled glass fiber reinforced epoxy matrix. The manufacturing process for developing this unique microstructure was developed. A total of seven groups of beam specimens with varying compositions were prepared. Each group contained 12 identical specimens with dimensions 304.8 mm × 50.8 mm × 15.2 mm. The total number of specimens was 84. Among them, 42 beams were pure foam core specimens and the remaining 42 beams were sandwich specimens with each foam core wrapped by two layers of E-glass plain woven fabric reinforced epoxy skin. Both low velocity impact tests and four-point bending tests were conducted on the foam cores and sandwich beams. Compared with the control specimens, the test results showed that the rubberized syntactic foams were able to absorb a considerably higher amount of impact energy with an insignificant sacrifice in strength. This multi-phase material contained structures bridging over several length-scales. SEM pictures showed that several mechanisms were activated to collaboratively absorb impact energy, including microballoon crushing, interfacial debonding, matrix microcracking, and fiber pull-out; the rubber layer and the microfibers prevented the microcracks from propagating into macroscopic damage by means of rubber pinning and fiber bridge-over mechanisms. The micro-length scale damage insured that the sandwich beams retained the majority of their strength after the impact.     

Full-field shear analyses of sandwich core materials using Digital Image Correlation (DIC)

Mechanical properties and global stability of foam core sandwich structures are highly controlled by the shear response of the core material. In this work, we have studied the shear deformations of three common structural core materials with the aid of full-field optical analysis. The chosen core materials are namely extruded PET foam (ρ = 105 kg/m³, G_xz = 21 MPa,) and cross-linked PVC foam (ρ = 60 kg/m³, G_xz = 22 MPa) which have comparable shear properties, as well as Balsa wood with the lowest density commercially available (ρ = 94 kg/m³, G_xz = 106 MPa) as a reference core material. Both global and local shear strains in the core materials are calculated and graphically visualized. In the elastic region, foam cores showed more uniform deformations than Balsa. Yielding and shear failure of the two foam core materials were quite different. The PVC foam experienced a high local deformation under the load introduction bars, from which sub-interface shear failure initiated. The PET foam, in contrast, showed no sign of stress concentrations, resulting in a homogenous evolution of shear deformations in the mid-core regions. A comparison between the direct foam shear test and sandwich specimen bending suggested that the former method might not be capable of capturing a full picture of the in-service core shear response.     

Characterization of electromagnetic properties of polymeric composite materials with free space method

The introduction of microwave radars during the second World War altered the air defense scenario significantly, and this led to the development of the “stealth” techniques. By reducing the detectability of aircrafts or warships, of which the radar cross section (RCS) is a measure, they could evade radar detection, which affected not only the mission success rate but also survival of them in the hostile territory. In the very early stage of the research on stealth techniques, many researches were mainly concentrated on the reduction of RCS and development of radar absorbing materials (RAM), but nowadays studies on investigating the radar absorbing structures (RAS) using fiber reinforced polymeric composite materials are becoming popular research field.

In this study, electromagnetic characteristics of unidirectional E-glass fiber reinforced epoxy composites were tested with free space methods, which can overcome drawbacks of conventional cavity and waveguide methods. Complex relative permittivities of low-loss composite were measured with respect to the angle between the fiber orientation and the electric field vector of EM wave in X-band frequency range. From the experimental data, empirical relation between the dielectric properties of composites and test variable was suggested and verified.     

Study on microwave absorption properties of metal-containing foam glass

Materials with the properties of electromagnetic (EM) wave absorption are attractive topics. In this work, we report that EM wave absorption composites, consisting of foam glass, zinc and zinc oxide, were prepared by sintering mixture of foam glass raw material and zinc powder. Microwave reflection loss of composite was calculated based on the permittivity in the range of 8.2-12.4 GHz. The results show that zinc-containing foam glass absorbs efficiently microwaves. The sample with zinc filler to foam glass mass ratio of 3/18 had a reflection loss below −10 dB in the range of 11.3-12.4 GHz, and the minimum reflectivity was −15.6 dB at both 12.0 and 12.4 GHz. Microwave absorption performances of specimens can be controlled by changing the ratio between zinc powder and foam glass mass. The detailed mechanism of the control was investigated through X-ray diffraction (XRD) analysis and scanning electrical microscopy (SEM) observations.